Babesial organisms are tick-transmitted intraerythrocytic parasites that are collectively called piroplasms, because they form pear-shaped figures within infected red blood cells. The piroplasms are almost universally transmitted by ixodid ticks and are capable of infecting a wide variety of vertebrate hosts that serve as reservoirs within the transmission cycle. Babesiosis is caused by infection with intraerythrocytic parasites of the genus Babesia. Most of what is known about the natural history of babesial infections comes from observations of vertebrate hosts other than humans, although the advent of new diagnostic tools has resulted in recent recognition of human babesiosis as an important tick-borne zoonosis. Human babesiosis was first described only in 1957 but is now known to have a worldwide distribution. The increase in reported cases is likely due to both increases in actual incidence and increased awareness of the disease. Despite improved understanding of the disease, babesiosis continues to have significant medical impact. It can also be a confounding variable in the diagnosis and treatment of Lyme disease and as a potential threat to the blood supply, especially in the United States. Diagnostic advances, including the development of PCR assays, have resulted in increased sensitivity of detection as well as the discovery and characterization of new babesial species. Further studies using the molecular tools available and those under development will lead to a better understanding of the natural history of this disease and its pathogenesis in humans.

Phylogenetic tree representation of a neighbor-joining analysis of several species of piroplasms. Five hundred nucleotides of the nss-rRNA were aligned using the Pileup program (Genetics Computer Group, University of Wisconsin). Phylogenetic analysis of the alignment was performed as described previously (101) using the Molecular Evolutionary Genetics Analysis (MEGA) computer program, version 1.01 (108), to make a Jukes-Cantor distance measurement and perform a neighbor-joining analysis with 500 bootstrap replicates. The Phylogenetic Analysis Using Parsimony (PAUP) computer program, version 3.1.1, was used to confirm the order observed by the neighbor-joining analysis (using a branch-and-bound algorithm with 100 bootstrap replicates).The percentage of neighbor-joining bootstrap replications (>50%) is shown above each node. This tree is consistent with previously published analyses. Species known to infect humans are marked with asterisks. The groups of large and small Babesia species are bracketed and labeled.

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Figure 1

Phylogenetic tree representation of a neighbor-joining analysis of several species of piroplasms. Five hundred nucleotides of the nss-rRNA were aligned using the Pileup program (Genetics Computer Group, University of Wisconsin). Phylogenetic analysis of the alignment was performed as described previously (101) using the Molecular Evolutionary Genetics Analysis (MEGA) computer program, version 1.01 (108), to make a Jukes-Cantor distance measurement and perform a neighbor-joining analysis with 500 bootstrap replicates. The Phylogenetic Analysis Using Parsimony (PAUP) computer program, version 3.1.1, was used to confirm the order observed by the neighbor-joining analysis (using a branch-and-bound algorithm with 100 bootstrap replicates).The percentage of neighbor-joining bootstrap replications (>50%) is shown above each node. This tree is consistent with previously published analyses. Species known to infect humans are marked with asterisks. The groups of large and small Babesia species are bracketed and labeled.

Life cycle of Babesia spp. in the tick and vertebrate hosts. Events in the tick begin with the parasites still visible in consumed erythrocytes. (A) Some are beginning to develop Strahlenkörper forms. (B) The released gametes begin to fuse (note that only one of the proposed mechanisms is pictured; one gamete has a Strahlenkörper form whereas the other does not). (C) The zygote then goes on to infect other tissues within the tick and migrates to the salivary glands. (D) Once a parasite has infected the salivary acini, a multinucleate but undifferentiated sporoblast is formed. (E) After the tick begins to feed, the specialized organelles of the future sporozoites form. (F) Finally, mature sporozoites bud off from the sporoblast. (G) As the tick feeds on a vertebrate host, these sporozoites are inoculated into the host. (H) Sporozoites (or merozoites) contact a host erythrocyte and begin the process of infection by invagination. (I) The parasites become trophozoites and can divide by binary fission within the host erythrocyte, creating the various ring forms and crosses seen on stained blood smears.

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Figure 2

Life cycle of Babesia spp. in the tick and vertebrate hosts. Events in the tick begin with the parasites still visible in consumed erythrocytes. (A) Some are beginning to develop Strahlenkörper forms. (B) The released gametes begin to fuse (note that only one of the proposed mechanisms is pictured; one gamete has a Strahlenkörper form whereas the other does not). (C) The zygote then goes on to infect other tissues within the tick and migrates to the salivary glands. (D) Once a parasite has infected the salivary acini, a multinucleate but undifferentiated sporoblast is formed. (E) After the tick begins to feed, the specialized organelles of the future sporozoites form. (F) Finally, mature sporozoites bud off from the sporoblast. (G) As the tick feeds on a vertebrate host, these sporozoites are inoculated into the host. (H) Sporozoites (or merozoites) contact a host erythrocyte and begin the process of infection by invagination. (I) The parasites become trophozoites and can divide by binary fission within the host erythrocyte, creating the various ring forms and crosses seen on stained blood smears.

Model of the cells and effector molecules involved in immunity to Babesia species. Different immune mechanisms contribute to resistance during each stage of the babesial infection. (A) During the establishment stage antibodies (IgG) play a role in preventing erythrocyte infection by binding the free sporozoites. (B) During the progression stage the Babesia organisms succeed in invading the erythrocyte, and the resulting merozoites proliferate and lyse the infected cell. After lysis has occurred, parasites reach the bloodstream again to initiate a new round of invasion. Several rounds of this cycle cause parasitemia levels to increase. Cells of the innate immune system, especially NK cells and macrophages, have been implicated in antibabesial activity and are thought to control the growth rate of the merozoites. The inhibition seems to rely on the production of soluble factors: IFN-γ by NK cells, and TNF-α, nitric oxide (NO), and ROS by macrophages. (C) In the resolution stage parasitemia reaches a maximum and then declines. The decrease in parasites seems to be due at least in part to intracellular degeneration inside the erythrocyte, as evidenced by the appearance of “crisis forms.” T lymphocytes seem to be the cells responsible for parasite clearance, specifically the subpopulation of CD4+ IFN-γ producers, although the specific role of IFN-γ is uncertain.

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Figure 3

Model of the cells and effector molecules involved in immunity to Babesia species. Different immune mechanisms contribute to resistance during each stage of the babesial infection. (A) During the establishment stage antibodies (IgG) play a role in preventing erythrocyte infection by binding the free sporozoites. (B) During the progression stage the Babesia organisms succeed in invading the erythrocyte, and the resulting merozoites proliferate and lyse the infected cell. After lysis has occurred, parasites reach the bloodstream again to initiate a new round of invasion. Several rounds of this cycle cause parasitemia levels to increase. Cells of the innate immune system, especially NK cells and macrophages, have been implicated in antibabesial activity and are thought to control the growth rate of the merozoites. The inhibition seems to rely on the production of soluble factors: IFN-γ by NK cells, and TNF-α, nitric oxide (NO), and ROS by macrophages. (C) In the resolution stage parasitemia reaches a maximum and then declines. The decrease in parasites seems to be due at least in part to intracellular degeneration inside the erythrocyte, as evidenced by the appearance of “crisis forms.” T lymphocytes seem to be the cells responsible for parasite clearance, specifically the subpopulation of CD4+ IFN-γ producers, although the specific role of IFN-γ is uncertain.

(A) Adult female lone star tick (A. americanum). The lone star tick is broadly distributed throughout the southeastern quadrant of the United States, with range extensions into New England and midwestern states. All three stages bite humans. A. americanum is a recognized vector of several pathogens that cause diseases in humans, including ehrlichioses caused by E. chaffeensis and E. ewingii, and has been implicated as a vector of southern tick-associated rash illness believed to be caused by “B. lonestari.” Other pathogens or potential pathogens have been detected in this tick, including various spotted fever group rickettsiae, F. tularensis, and C. burnetii, although the role of A. americanum in the transmission of these agents to humans is not well characterized. (B) Adult female American dog tick (D. variabilis). This tick is abundant in the southeastern United States and coastal New England, with limited distribution in several midwestern states, southern Canada, and western coastal regions. The tick is best known as the primary vector of Rocky Mountain spotted fever in the eastern United States. (C) Adult female blacklegged or deer tick (I. scapularis). As a member of the greater I. persulcatus group that also includes I. pacificus, I. ricinus, and I. persulcatus, this tick is an important vector of several infectious agents that cause disease in humans, including Lyme borreliosis, (granulocytic) anaplasmosis, and babesiosis. I. persulcatus ticks elsewhere in the world may also transmit tick-borne flaviviruses capable of causing encephalitis, encephalomyelitis, and even hemorrhagic fevers (including tick-borne encephalitis virus, louping ill virus, Russian spring-summer encephalitis virus, Powassan virus, Kyasanur Forest disease virus, and Langat virus). Images courtesy of G. Maupin.

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(A) Adult female lone star tick (A. americanum). The lone star tick is broadly distributed throughout the southeastern quadrant of the United States, with range extensions into New England and midwestern states. All three stages bite humans. A. americanum is a recognized vector of several pathogens that cause diseases in humans, including ehrlichioses caused by E. chaffeensis and E. ewingii, and has been implicated as a vector of southern tick-associated rash illness believed to be caused by “B. lonestari.” Other pathogens or potential pathogens have been detected in this tick, including various spotted fever group rickettsiae, F. tularensis, and C. burnetii, although the role of A. americanum in the transmission of these agents to humans is not well characterized. (B) Adult female American dog tick (D. variabilis). This tick is abundant in the southeastern United States and coastal New England, with limited distribution in several midwestern states, southern Canada, and western coastal regions. The tick is best known as the primary vector of Rocky Mountain spotted fever in the eastern United States. (C) Adult female blacklegged or deer tick (I. scapularis). As a member of the greater I. persulcatus group that also includes I. pacificus, I. ricinus, and I. persulcatus, this tick is an important vector of several infectious agents that cause disease in humans, including Lyme borreliosis, (granulocytic) anaplasmosis, and babesiosis. I. persulcatus ticks elsewhere in the world may also transmit tick-borne flaviviruses capable of causing encephalitis, encephalomyelitis, and even hemorrhagic fevers (including tick-borne encephalitis virus, louping ill virus, Russian spring-summer encephalitis virus, Powassan virus, Kyasanur Forest disease virus, and Langat virus). Images courtesy of G. Maupin.

(A to E) Soft tick morphology. (A) The soft tick O. moubata is long lived and feeds repeatedly on humans and other mammals. (A and E) Dorsal aspect of an engorged female: the cuticle has a leather-like texture, lending to the name “Lederzecke” (leather tick), the German vernacular for argasid ticks. (B, C, and D) The capitulum is located in a subterminal, ventral position. The unfed argasid ticks are flat and highly mobile. When inactive, they hide in cracks and crevices. Each blood meal takes minutes to a few hours. In contrast, adult ixodid females feed for several days. (F to I) Hard tick morphology. (F) Engorged female A. americanum tick. (G) Closeup of mouthparts I. scapularis nymph. C, chelicerae; P, palps; H, hypostome. The mobile distal cheliceral teeth reach beyond the hypostome. (H) During feeding, the palps are splayed laterally (female D. andersoni tick). (I) A feeding D. andersoni couple; the female tick is in front. (J) Tick feeding lesion. Adult I. scapularis female (below) feeding on a sensitized rabbit (above). Predominantly mixed inflammatory infiltrate cells line the feeding cavity. Discrete dermal swelling and dilated venules are found in the proximity. The mouthparts are anchored deep into the dermis, just reaching the cavity from which the tick feeds as indicated by the imprint of the hypostome contours (trichrome stain). Photographs courtesy of S. Archibald and E. Denison.

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(A to E) Soft tick morphology. (A) The soft tick O. moubata is long lived and feeds repeatedly on humans and other mammals. (A and E) Dorsal aspect of an engorged female: the cuticle has a leather-like texture, lending to the name “Lederzecke” (leather tick), the German vernacular for argasid ticks. (B, C, and D) The capitulum is located in a subterminal, ventral position. The unfed argasid ticks are flat and highly mobile. When inactive, they hide in cracks and crevices. Each blood meal takes minutes to a few hours. In contrast, adult ixodid females feed for several days. (F to I) Hard tick morphology. (F) Engorged female A. americanum tick. (G) Closeup of mouthparts I. scapularis nymph. C, chelicerae; P, palps; H, hypostome. The mobile distal cheliceral teeth reach beyond the hypostome. (H) During feeding, the palps are splayed laterally (female D. andersoni tick). (I) A feeding D. andersoni couple; the female tick is in front. (J) Tick feeding lesion. Adult I. scapularis female (below) feeding on a sensitized rabbit (above). Predominantly mixed inflammatory infiltrate cells line the feeding cavity. Discrete dermal swelling and dilated venules are found in the proximity. The mouthparts are anchored deep into the dermis, just reaching the cavity from which the tick feeds as indicated by the imprint of the hypostome contours (trichrome stain). Photographs courtesy of S. Archibald and E. Denison.

Anatomy of a tick bite. This series of images was obtained from a single feeding adult female A. americanum tick removed by biopsy from a patient in Maryland. The images show the partially engorged tick embedded in the skin (A) and the tick and skin sectioned en block (B) (hematoxylin and eosin [H&E] stain; original magnification, ×8). (C to E) Positioning of the tick capitulum and hypostome with relation to the epidermis (H&E stain; original magnification ×16) (C and D), the positioning of the hypostome into the dermis with the inflammatory vascular pool at its distal end (H&E stain; original magnification, ×16) (E), and the inflammation and edema present in the dermal blood pool upon which the tick feeds (H&E stain; original magnification, ×40). (F) Anatomical structures of importance in acquisition and transmission of pathogens by ticks with subsequent molts include the midgut into which pathogens pass with the blood meal and the hemocele into which pathogens pass after penetration of the midgut epithelium (H&E stain; original magnification, ×40). (G) Pathogens that are transmitted by tick bite gain access to the tick salivary gland before passing into the dermal vascular pool in tick saliva (H&E stain; original magnification, ×40). Not shown is the tick ovary, into which some pathogens may invade to allow transovarian transmission.

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Anatomy of a tick bite. This series of images was obtained from a single feeding adult female A. americanum tick removed by biopsy from a patient in Maryland. The images show the partially engorged tick embedded in the skin (A) and the tick and skin sectioned en block (B) (hematoxylin and eosin [H&E] stain; original magnification, ×8). (C to E) Positioning of the tick capitulum and hypostome with relation to the epidermis (H&E stain; original magnification ×16) (C and D), the positioning of the hypostome into the dermis with the inflammatory vascular pool at its distal end (H&E stain; original magnification, ×16) (E), and the inflammation and edema present in the dermal blood pool upon which the tick feeds (H&E stain; original magnification, ×40). (F) Anatomical structures of importance in acquisition and transmission of pathogens by ticks with subsequent molts include the midgut into which pathogens pass with the blood meal and the hemocele into which pathogens pass after penetration of the midgut epithelium (H&E stain; original magnification, ×40). (G) Pathogens that are transmitted by tick bite gain access to the tick salivary gland before passing into the dermal vascular pool in tick saliva (H&E stain; original magnification, ×40). Not shown is the tick ovary, into which some pathogens may invade to allow transovarian transmission.

The ulceroglandular form of tularemia (see Chapter 12) may present with a striking ulcer, such as that seen here on a finger (A) or with lymphadenitis, which may also occur independently without evidence of an ulcer (so-called glandular tularemia) or in oropharyngeal tularemia (B). (C) Affected lymph glands are characterized by multiple granulomas and geographic necrotizing inflammation (H&E stain; original magnification, ×12.5). (D) The lymph node shows granulomas and granulomatous inflammation with palisaded histiocytes surrounding an extensive central region of suppurative and necrotizing inflammation (H&E stain; original magnification, ×100). (E) F. tularensis can be demonstrated by immunohistochemistry within histiocytes in the necrotic tissue (original magnification, ×100). Images in panels D and E courtesy of S. R. Zaki.

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The ulceroglandular form of tularemia (see Chapter 12) may present with a striking ulcer, such as that seen here on a finger (A) or with lymphadenitis, which may also occur independently without evidence of an ulcer (so-called glandular tularemia) or in oropharyngeal tularemia (B). (C) Affected lymph glands are characterized by multiple granulomas and geographic necrotizing inflammation (H&E stain; original magnification, ×12.5). (D) The lymph node shows granulomas and granulomatous inflammation with palisaded histiocytes surrounding an extensive central region of suppurative and necrotizing inflammation (H&E stain; original magnification, ×100). (E) F. tularensis can be demonstrated by immunohistochemistry within histiocytes in the necrotic tissue (original magnification, ×100). Images in panels D and E courtesy of S. R. Zaki.

Ehrlichia spp. infections include human monocytic ehrlichiosis (HME) (E. chaffeensis) (see Chapter 14) and infection by E. ewingii (see Chapter 15). (A) Typical E. chaffeensis morula with pale blue violaceous bacteria in a circulating blood monocyte (Wright stain; original magnification, ×400). Image courtesy of Aileen Marty. (B) Morulae of E. chaffeensis isolated from the blood of a patient with HME growing in the DH82 canine macrophage cell line. (C) As with human granulocytic anaplasmosis (HGA), HME frequently involves the liver and may be severe. The image shows necrosis and moderate lobular hepatitis (H&E; original magnification, ×16). (D) Small noncaseating granulomas typical of those frequently seen in tissues of patients with HME, including the bone marrow (H&E stain; original magnification, ×16). (E) Meningitis and meningoencephalitis occur in approximately 20% of patients with HME but are rare in HGA. Shown is a mild infiltration of the meninges by lymphocytes and histiocytes in a patient who died after contracting HME (H&E; original magnification, ×16). (F) As with A. phagocytophilum, E. ewingii propagates in neutrophils in peripheral blood (modified Wright's stain; original magnification, ×400); (G) it may also occasionally be identified in inflammatory infiltrates in various organs, as in this bronchoalveolar lavage specimen of a patient with pulmonary manifestations associated with E. ewingii infection (modified Wright's stain, original magnification, ×400).

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Ehrlichia spp. infections include human monocytic ehrlichiosis (HME) (E. chaffeensis) (see Chapter 14) and infection by E. ewingii (see Chapter 15). (A) Typical E. chaffeensis morula with pale blue violaceous bacteria in a circulating blood monocyte (Wright stain; original magnification, ×400). Image courtesy of Aileen Marty. (B) Morulae of E. chaffeensis isolated from the blood of a patient with HME growing in the DH82 canine macrophage cell line. (C) As with human granulocytic anaplasmosis (HGA), HME frequently involves the liver and may be severe. The image shows necrosis and moderate lobular hepatitis (H&E; original magnification, ×16). (D) Small noncaseating granulomas typical of those frequently seen in tissues of patients with HME, including the bone marrow (H&E stain; original magnification, ×16). (E) Meningitis and meningoencephalitis occur in approximately 20% of patients with HME but are rare in HGA. Shown is a mild infiltration of the meninges by lymphocytes and histiocytes in a patient who died after contracting HME (H&E; original magnification, ×16). (F) As with A. phagocytophilum, E. ewingii propagates in neutrophils in peripheral blood (modified Wright's stain; original magnification, ×400); (G) it may also occasionally be identified in inflammatory infiltrates in various organs, as in this bronchoalveolar lavage specimen of a patient with pulmonary manifestations associated with E. ewingii infection (modified Wright's stain, original magnification, ×400).

Rocky Mountain spotted fever (RMSF). R. rickettsii infects and damages endothelial cells and may cause profound vascular leakage. A macular or maculopapular rash that blanches with pressure is typically detected in the first several days of illness. (A) Petechiae frequently develop as the rash evolves, as a result of extravasation of erythrocytes into the dermis where vasculitis (C) may also be observed (H&E stain; original magnification, ×8). (C, inset) The rickettsiae alone are sufficient to cause vascular leakage, but their presence also elicits lymphohistiocytic and leukocytoclastic vasculitis, occasionally associated with fibrin deposition or non-occlusive thrombus formation (H&E stain; original magnification, ×16). (E) Vasculitis may involve all organs, and the most significant complications include pulmonary and central nervous system involvement (R. rickettsii immunohistochemistry; original magnification, ×260). (B) Pulmonary involvement may lead to bilateral interstitial infiltrates seen on chest radiographs. (D) Markedly increased microvascular permeability in the lung, believed to be due to rickettsia-mediated endothelial cell damage (H&E stain; original magnification, ×16) may lead to noncardiogenic pulmonary edema (R. rickettsii immunohistochemistry in skin; original magnification, ×260) (F). Meningoencephalitis and rickettsia-induced endothelial cell damage and inflammation can lead to cerebral edema and herniation. (G to I) Meningeal vasculitis (H&E stain; original magnification, ×100) (G) and one of many scattered mononuclear inflammatory cell infiltrates in the brain called microglial or typhus nodules (H&E stain; original magnification, ×100) (H). Immunohistochemical staining for R. rickettsii shows widespread rickettsial infection of cerebral microvascular endothelial cells (original magnification, ×160) (I).

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Rocky Mountain spotted fever (RMSF). R. rickettsii infects and damages endothelial cells and may cause profound vascular leakage. A macular or maculopapular rash that blanches with pressure is typically detected in the first several days of illness. (A) Petechiae frequently develop as the rash evolves, as a result of extravasation of erythrocytes into the dermis where vasculitis (C) may also be observed (H&E stain; original magnification, ×8). (C, inset) The rickettsiae alone are sufficient to cause vascular leakage, but their presence also elicits lymphohistiocytic and leukocytoclastic vasculitis, occasionally associated with fibrin deposition or non-occlusive thrombus formation (H&E stain; original magnification, ×16). (E) Vasculitis may involve all organs, and the most significant complications include pulmonary and central nervous system involvement (R. rickettsii immunohistochemistry; original magnification, ×260). (B) Pulmonary involvement may lead to bilateral interstitial infiltrates seen on chest radiographs. (D) Markedly increased microvascular permeability in the lung, believed to be due to rickettsia-mediated endothelial cell damage (H&E stain; original magnification, ×16) may lead to noncardiogenic pulmonary edema (R. rickettsii immunohistochemistry in skin; original magnification, ×260) (F). Meningoencephalitis and rickettsia-induced endothelial cell damage and inflammation can lead to cerebral edema and herniation. (G to I) Meningeal vasculitis (H&E stain; original magnification, ×100) (G) and one of many scattered mononuclear inflammatory cell infiltrates in the brain called microglial or typhus nodules (H&E stain; original magnification, ×100) (H). Immunohistochemical staining for R. rickettsii shows widespread rickettsial infection of cerebral microvascular endothelial cells (original magnification, ×160) (I).

Human babesiosis (see Chapter 20) can be caused by several different protozoan species in the genus Babesia. In North America, most cases of human babesiosis are caused by B. microti. (A) B. microti forms pleomorphic and amoeboid intraerythrocytic rings that can be difficult to differentiate from other babesial species and from Plasmodium spp. (Wright stain, original magnification, ×160). (B) Rare examples of infection by B. gibsoni-like piroplasms called WA-1 (Wright stain; original magnification, ×160) have been documented in the Pacific northwest and in California. Image courtesy of D. H. Persing. (C) B. divergens-like piroplasms are found in Europe and have also been observed in an infected person in Missouri in the south-central United States (Wright stain; original magnification, ×160). Image courtesy of E. Masters. When present, a helpful finding differentiating Babesia spp. from Plasmodium spp. is the presence of tetrads of merozoites, termed Maltese crosses (B). (D) B. microti in a peripheral blood smear from an infected patient detected by acridine orange staining and fluorescent microscopy. Image courtesy of D. H. Persing.

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Human babesiosis (see Chapter 20) can be caused by several different protozoan species in the genus Babesia. In North America, most cases of human babesiosis are caused by B. microti. (A) B. microti forms pleomorphic and amoeboid intraerythrocytic rings that can be difficult to differentiate from other babesial species and from Plasmodium spp. (Wright stain, original magnification, ×160). (B) Rare examples of infection by B. gibsoni-like piroplasms called WA-1 (Wright stain; original magnification, ×160) have been documented in the Pacific northwest and in California. Image courtesy of D. H. Persing. (C) B. divergens-like piroplasms are found in Europe and have also been observed in an infected person in Missouri in the south-central United States (Wright stain; original magnification, ×160). Image courtesy of E. Masters. When present, a helpful finding differentiating Babesia spp. from Plasmodium spp. is the presence of tetrads of merozoites, termed Maltese crosses (B). (D) B. microti in a peripheral blood smear from an infected patient detected by acridine orange staining and fluorescent microscopy. Image courtesy of D. H. Persing.

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